Kubiiki Pride is the mother of one of the exploited girls whose story is told in “I Am Jane Doe.” (Richard Schultz/50 Eggs Films)

The documentary “I Am Jane Doe” is the kind of film that lifts up a rock that’s been sitting in plain sight year after year, with only a heroic few bothering to see the slithering reality underneath. Now, thanks to this sobering, often sickening, bluntly effective piece of advocacy cinema, the rest of us have no excuse to turn away.

The subject at hand is the sexual exploitation of children — specifically, thousands of girls, often runaways, who have been tricked or manipulated into becoming “prostitutes” (or, more accurately, rape victims and enslaved sex workers). Historically, the chief outlet that their pimps advertised on was a website called Backpage.com, a Craigslist-like classified bulletin board that, along with information about bikes for sale and apartments for rent, ran a booming business in the “escort” industry, accounts payable by credit card or bitcoin.

In “I Am Jane Doe,” filmmaker Mary Mazzio tracks down some of the girls who managed to survive their ordeals and the families — mostly mothers — who filed lawsuits against Backpage for conspiring in the illegal activities of their pimps and abusers.

Relating a few pivotal lawsuits filed in St. Louis, Seattle and Boston, narrator Jessica Chastain (also one of the executive producers) threads viewers through a Kafkaesque labyrinth of legal dodges and breathtaking judicial cynicism, most of which centers on an obscure clause in the Communications Decency Act. (Enacted in 1996, the CDA protected Internet service providers from liability for material published on their platforms by third parties.)

One of the victims of Backpage, identified in the film only as “JS.” (Richard Schultz/50 Eggs Films)

As it happens, Backpage was once owned by the Village Voice, which would seem to have put it squarely in the crosshairs of the kind of aggressive investigative reporting and righteous indignation for which that venerable New York newspaper earned its journalistic bones. The editorial inaction of the Voice — even as Backpage grew steadily sleazier — is just one of the ironies that abound in “I Am Jane Doe,” which also features cameos from such famous jurists as Richard Posner and David Souter, as well as high-profile litigator David Boies.

But the heroes of the film are the young Jane Does who courageously share their stories, along with their tireless parents and the local attorneys who have clearly made it their mission to find redress for their clients and amend the CDA, after being continually swatted down by courts with seemingly endless capacity for denial of the law’s real-world consequences.

“I Am Jane Doe” became unexpectedly timely last month, when the Senate Permanent Subcommittee on Investigations finally succeeded in questioning current and former Backpage executives, who repeatedly claimed their Fifth Amendment rights against self-incrimination. The day before, they had shut down its “adult services” section, claiming government censorship. And indeed, Backpage has enlisted powerful allies in its free-speech argument, including the Electronic Frontier Foundation and the Cato Institute — as well as some sex workers and nonprofits who argue that Backpage was actually helpful in making their work safer, and in identifying exploitative perpetrators.

Those views aren’t explored in “I Am Jane Doe,” which doesn’t apologize for voicing only one side of the debate but nonetheless makes a viscerally emotional case for a common-sense reassessment of the law. With luck, the conversation will continue, in Congress and elsewhere. If anything, “I Am Jane Doe” offers a gut-wrenching reminder that there are certain rocks we ignore at our peril.